Jet streams are narrow bands of high-speed winds that move like great rivers of air around Earth in each hemisphere. These rivers of air are located between 8 to 12 miles (13 to 19 km) above Earth's surface, and generally flow from west to east at mid-latitudes. Wind speeds within jet streams reach as high as 250 miles per hour (400 km/hr).

National Weather
Service

Generalized view of
the polar jet stream flowing across North America.

Notice
that the fastest wind speeds tend to be in the middle
of the jet stream.